This invention generally involves rolling cutter drill bits and more specifically involves an improved journal configuration for rolling cutter drill bits. The present invention, while being advantageous for unsealed roller bearing bits, is also advantageous in sealed roller bearing bits and in sealed and non-sealed friction bearing bits.
During the operation of the prior art drill bits the rolling cutter rotates on a machined journal extending downward from the lug. The journal generally comprises a radial bearing surface which is the main bearing, and a retention or thrust bearing which is a ball bearing. In addition, the journal has a smaller machined section at the lowermost end which is a pilot pin and which also has a secondary radial bearing surface machined thereon preferably concentric with the main radial bearing surface. During operation of the prior art drill bit the rolling cutter rotates on the two radial bearing surfaces described above which surfaces receive the main radial bearing load. The forces attempting to push the rolling cutter upward on the journal or pull the cutter downward from the journal are termed axial or thrust forces and are sometimes shared by the ball bearing, the perpendicular end of the pilot pin, and the annular surface of the end of the main journal, surrounding the pilot pin perpendicular to the radial bearing surfaces. In some bits only the downwardly directed axial forces are loaded on the ball bearings and the upward thrust forces are carried by the two perpendicular surfaces.
Because the concentric surfaces of the main journal section and the pilot pin section both conjointly share the radial bearing load, the surfaces preferably are machined to extremely close tolerances. This requires a very exacting machine operation because of the non-contiguous bearing surfaces on the main journal and the pilot pin. As a result of the failure to machine these two surfaces exactly concentric, which failure must arise through normal machining tolerances, the rolling cutter under various loading conditions will become cocked on the journal. This is because the tolerances between the main radial bearing surface and the pilot pin bearing surface are additive and allow the tilting of the cutter with respect to the surfaces. This tilting results in a single point contact between the bearing surfaces on the journal and the mating surfaces of the bearing and the cutter internal surface. This point loading results in extremely rapid wear from spalling and galling between the journal and the cutter. In order to remove the accumulated tolerances between the parallel surfaces of the main bearing surface and the pilot pin bearing surface, extremely complex and expensive machine operations would be required. Because of the economics involved, these machining operations are not usually performed in the drill bit manufacture.
In addition to the problems encountered with machining the non-contiguous radial bearing surfaces to reduce eccentricities therebetween, there is also a problem with axial thrust bearing surfaces in prior art devices. A majority of the conventional drill bits utilize the perpendicular end of the pilot pin as a thrust load sharing surface. In manufacturing the cutter to receive the pilot pin, a weakened area is formed in the cutter cone by the recess machined into it for the pilot pin. This weakness has resulted in the pin punching through the cone during drilling operations, and has caused loss of the hard cone in the borehole. This requires an expensive trip out of the hole and a fishing operation to clear the ruptured cone from the borehole.
Thus, the standard drill bit manufactured today results in the accumulated tolerances which leads to excessive wear through point loading contact, and weakened cone structure due to the pilot pin construction. The present invention greatly reduces the problem of accumulated tolerances by providing a journal on a drill bit lug which has a single radial bearing surface and a large flat thrust bearing surface on the end of the journal perpendicular to the main radial bearing surface. This eliminates the pilot pin configuration which consequently eliminates the accumulated tolerances between the main radial bearing surface and the pilot pin radial bearing surface. Thus, the present invention greatly reduces the premature and rapid deterioration of the bearing surfaces due to the cocking of the rolling cutter on the journal of the bit lug, and eliminates weakness in the cutter cone from the pilot pin recess in the conventional bit.